The increasing demand for higher performance liquid crystal display (LCD) has resulted in a continuous development of inverter controllers for cold cathode fluorescent lamps (CCFL) and incorporation of such inverter controllers into integrated circuits. Many LCD applications, such as in notebook, LCD monitor, LCD TV and other display devices, require the use of an inverter controller with high-efficiency to drive the CCFL. These LCD applications typically require fast response to variations of a supply voltage and good driving signals with fixed frequency and desirable duty cycle to increase the system efficiency and longevity of the LCD applications.
Generally, the inverter controller can provide a pulse width modulation (PWM) signal with a certain frequency and a duty cycle to an inverter circuit. The inverter circuit can convert a DC signal such as the supply voltage, into an alternating current (AC) signal to supply power to drive a plurality of loads, such as the CCFL in various display applications. The inverter controller is usually configured to control the required power to ignite the CCFL through the inverter circuit. In order to provide the required power to the loads, the inverter controller is required to adjust its output signal when the supply voltage varies under various conditions.
The inverter controller typically consists of an error amplifier, a comparator for PWM, and a driver. These three components are coupled in series. A compensation capacitor can be coupled between an output terminal of the error amplifier and an input terminal of the comparator. In addition, the inverter controller is coupled in series with the inverter circuit to generate a desirable signal to the loads. Conventionally, the inverter controller may respond to the variations of the supply voltage by regulating a PWM controlling signal at the output terminal of the error amplifier. However, the regulation speed of the inverter controller can be adversely influenced by charging or discharging the compensation capacitor that is connected to the output terminal of the error amplifier. Consequently, the variation of the PWM controlling signal under the control of a variable supply voltage will cause the duty cycle of the PWM signal to vary. The variation of the duty cycle of the PWM signal that will control the supply power to the CCFL through the inverter will have an inverse effect on the brightness of the CCFL.
The error amplifier can also be implemented with a bias current inversely proportional to the supply voltage to realize feed-forward compensation. However, it is difficult to design the integrated circuit with the bias current that is a precisely and inversely proportional to the supply voltage. In other words, the precision of the inverse proportionality between the bias current and the supply voltage makes the configuration of the integrated circuit more complex.
Thus, there is a need to overcome the above drawbacks and disadvantages in the prior art and to provide a circuitry solution with feed-forward compensation that features simple configuration, high efficiency, reliable ignition of the CCFL, and higher and precise frequency. Therefore, it is to such need the invention primarily directed.